1. Field of the Invention
The present invention generally relates to a micro-electro-mechanical systems (MEMS) structure, in particular, to a structure of MEMS electroacoustic transducer.
2. Description of the Prior Art
Electroacoustic transducers are capable of converting acoustic waves into electrical signals or converting electrical signals into acoustic waves by the use of diaphragms, so as to serve as the microphones or speakers. The electroacoustic transducers have wide applications, such as in mobile phone, digital camera, hands free phone, and notebook computer products in the computer communication industry, or in the medical instruments such as hearing aids and cochlear implant products.
Along with the rapid development of the electronic industry and progress of the semiconductor process and packaging technique, the electroacoustic transducer products are designed in the trend of multifunction. In order to satisfy the demands for “light, thin, short, small,” power-saving, and low price, it is the trend for the electroacoustic transducers to develop micro-electroacoustic transducers that may be integrated with chips fabricated through the semiconductor process.
Micro-electro-mechanical systems (MEMS) electroacoustic transducers are formed by designing the mechanical devices and electronic devices on chips through the integrated circuit technique. By way of example, in consideration of the current development, the MEMS microphones are designed based on the capacitive principles. The basic construction of a capacitive microphone mainly includes a electrode fixed on a soft diaphragm and a rigid back plate respectively, and a backside cavity formed between the diaphragm and the back plate, so as to achieve the freely vibration along with the sounds. The electric field between the vibrated diaphragm and the back plate varies, thus producing the electronic signals transmitted on the circuits.
Generally speaking, after the backside cavity is formed, a polymer layer is formed to seal the diaphragm, so as to create the air-tight environment. However, the material of the polymer layer is overly soft, which causes the problem that the sensitivity of the electroacoustic transducer is unsatisfactory. Furthermore, in practical operation, the method of sealing the diaphragm with the polymer material has complex procedures, and the quality of the polymer layer is not easy to control.